The hydrate of a neutral iron(<scp>iii</scp>) complex based on the pyruvic acid thiosemicarbazone ligand with abrupt spin-crossover with <i>T</i>_1/2 = 340 K and a wide hysteresis loop of 45 K

The hydrate of a neutral iron(III) complex based on the pyruvic acid thiosemicarbazone ligand [Fe^III(Hthpy)(thpy)]·H₂O (1) was synthesized and characterized using FT-IR spectroscopy, powder and single-crystal X-ray diffraction, <i>dc</i> magnetic measurements, EPR and ⁵⁷Fe Mössbauer spectroscopy. The crystal structure of 1 was determined for the first time. Two distinct chelating ligands Hthpy^- and thpy^2- coordinate the Fe(III) ion to form the FeN₂O₂S₂ octahedron which shows a low spin geometry at 150-350 K. The crystal packing contains infinite chains of the Fe(III) complexes as well as water molecules located in cavities. Along the chain, π-π interacting pairs of the Fe complexes are linked by H-bonding. According to the <i>dc</i> magnetic measurements, the complete abrupt spin-crossover with half-transition temperature <i>T</i>_1/2 = 340 K and a hysteresis loop of 45 K occurs in the temperature range of 300-363 K. Based on the X-ray structure of 1, the Bleaney-Bowers equation for the isolated dimer model was used to approximate the temperature dependence of the magnetic susceptibility in the range of 2-50 K. The defined intradimer exchange constant <i>J</i>_exp = -0.498(1) K corresponds to a weak antiferromagnetic exchange between the iron(III) magnetic centers. DFT calculations of H- and π-π bonded fragments of the crystal structure of 1 in the HS and LS states were carried out. Moreover, BS-DFT calculations confirm the presence of antiferromagnetic exchange <i>J</i>_calc = -0.92 K in the π-π bonded pairs of the ferric complexes and show the exchange pathway between Fe(III) ions by the calculated spin density distribution.